Almost every language has some sort of requirement for where you put files to properly compose a library. Dart is one of the sanest I know of, just throw your stuff in `lib/mylib.dart`. This file is normally only used to `export` what your lib API should look like, and it's a convention that the implementations go in `lib/src/`. That's all there is! You can break up files apart any way you want from there. To import a file from another file you do `import 'relative/path/to/file.dart`. Dart doesn't care where your imported files are as long as it can find them. There's no "packages" like in Java, but that's never a problem because imports can be named.
So you do stuff like:
import 'mylib.dart' as mylib;
// call mylib function
main() => mylib.foo();
I have looked at forking the language to explore an alternate system, but the path of least resistance has been using other languages.